a(n) = (n+2)!/4 + n!/2.

A006595

a(n) = (n+2)!/4 + n!/2.

Terms

    a(0) =1a(1) =2a(2) =7a(3) =33a(4) =192a(5) =1320a(6) =10440a(7) =93240a(8) =927360a(9) =10160640a(10) =121564800a(11) =1576713600a(12) =22034073600a(13) =330032102400

External references